1. Field of the Invention
The present invention relates to an optical system for shooting a three-dimensional image and a three-dimensional image shooting apparatus using the optical system, which are suitable for alternately introducing images having left and right parallaxes (parallax images) to one image pickup device in a time series by using a shutter means, thereby producing a three-dimensional image.
2. Description of the Related Art
A plurality of parallax images for the left and right eyes are necessary to observe a three-dimensional image. Heretofore, various optical apparatuses and optical systems for shooting a three-dimensional image have been proposed to provide a plurality of parallax images.
Of the proposed optical apparatuses and systems, a three-dimensional image shooting apparatus for alternately introducing images having left and right parallaxes to the same area of one image pickup device in a time series by using a liquid crystal shutter is proposed, for example, in Japanese Patent Laid-Open No. 06-327036.
The three-dimensional image shooting apparatus shown in FIG. 1 of Japanese Patent Laid-Open No. 06-327036 comprises two reflecting mirrors on each of left and right sides, a liquid crystal shutter capable of being driven in an area split manner, and a shooting lens. The liquid crystal shutter is arranged near the entrance pupil of the shooting lens on the side closer to a subject with respect to the shooting lens so that images having left and right parallaxes may alternately enter the one image pickup device.
The liquid crystal shutter used in the three-dimensional image shooting apparatus proposed in Japanese Patent Laid-Open No. 06-327036 generally has a characteristic that the extinction ratio deteriorate as shown in FIG. 4, by way of example, as the incident angle of a light beam deviates increasingly from a direction perpendicular to a polarizing plate.
FIG. 4 shows one example of actually measured values of the changes in the extinction ratio resulting when the incident angle is varied from 0 degree, at which the optical beam enters the liquid crystal shutter perpendicularly, to 20 degrees.
Here, the term xe2x80x9cextinction ratioxe2x80x9d means a ratio of the maximum value to a minimum value of transmittance of the liquid crystal shutter.
In FIG. 4, a solid line FLC represents an angle characteristic of the extinction ratio of a liquid crystal shutter employing a ferroelectric liquid crystal, and a dotted line TN represents an angle characteristic of the extinction ratio of a liquid crystal shutter employing a twisted nematic liquid crystal.
Generally, as shown in FIG. 4, the liquid crystal shutter employing a ferroelectric liquid crystal exhibits smaller changes of the extinction ratio depending on changes of the incident angle.
The characteristic of the liquid crystal shutter, shown in FIG. 4, raises the following problem. When the liquid crystal shutter is arranged on the side closer to a subject with respect to the shooting lens as with the above-described related art, the incident angle of a light beam entering the liquid crystal shutter is increased depending on the angle of view of the shooting lens. As a result, the greater the angle of view of the shooting lens used, the greater the amount of light leaked in peripheral portions of the image frame of the image pickup device when light is to be cut off.
Therefore, when images having left and right parallaxes are alternately introduced to enter one image pickup device in a time series as intended by the above conventional three-dimensional image shooting apparatus, an image on the side to be cut off is leaked in its peripheral portions. For example, when it is desired to input only a right-side image, peripheral portions of a left-side image appear faintly like a ghost image in overlapped relation to the right-side image, thus resulting in a very unsightly three-dimensional image.
Also, when a liquid crystal shutter is used as a light amount control means, light after passing the shutter is polarized. In the case of employing an image pickup device such as a CCD which takes in images periodically, therefore, the use of a low-pass filter utilizing double refraction weakens the low-pass effect to such an extent that monochromatic moirxc3xa9 and color moirxc3xa9 are produced. This has raised a problem of making a picked-up image unsightly.
Still another problem has been experienced in that, because the light received for shooting is also polarized by a polarizing plate of the liquid crystal shutter positioned on the side closer to a subject, the luster of the subject actually perceived by the naked eyes of a photographer is significantly different from the luster of a picked-up image of the subject.
A primary object of the present invention is to provide an optical system for shooting a three-dimensional image and a three-dimensional image shooting apparatus using the optical system, which can produce an image with a smaller amount of light leaked from the side to be cut off over an entire image frame when parallax images are obtained by employing a shutter, such as a liquid crystal shutter, in which an extinction ratio varies depending on the incident angle.
A secondary object of the present invention is to provide an optical system for shooting a three-dimensional image and a three-dimensional image shooting apparatus using the optical system, which can suppress the occurrence of monochromatic moirxc3xa9 and color moirxc3xa9, and which enables a subject to have natural luster in its picked-up image.
To achieve the above objects, the optical system of the present invention has the following features.
The optical system comprises a pair of front optical components each having negative optical power; an optical member for superimposing optical axes of the pair of front optical components with each other; a rear optical component disposed to have an optical axis aligned with the superimposed optical axes; and a shutter capable of independently controlling the amounts of transmitted light incident upon the pair of front optical components from a object, the shutter being disposed between the pair of front optical components and the rear optical component. The optical system forms parallax images in a time series by alternately changing the amounts of transmitted light incident upon the pair of front optical components from the subject with the shutter.
Also, the three-dimensional image shooting apparatus of the present invention comprises the optical system of the present invention, and an image pickup device disposed on an image focused plane of the optical system.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments( with reference to the attached drawings).